4 Answers
4

It takes more experience to gain the higher levels. As such, if you use Rare Candies on the early levels, you're pretty much wasting them as the experience they'd be worth on a higher level Pokémon is far greater than on a lower level one. Especially as you don't generally encounter Pokémon higher than level 70, which means your experience income doesn't quite reach all that high.

Your Pokémon gain bonuses to their stats based on fighting other Pokémon. This is what makes the whole "Trained pokémon are stronger than wild pokémon of the same level" point, is from this stat bonus. Generations 1-4, however, this stat bonus can only be realized when leveling up during actual battle. The system was changed in Generation 5 but this "must level during battle" factor is present in Blue.

The amount of bonus stats you can gain does have a cap, though. If you've been thoroughly training a Pokémon since the lower levels (or if you just fed them a bunch of stat-raising vitamins, which correspond to the bonus stats), you'll probably have hit that cap by the time you should start using Rare Candies.

In general, the above basically means to train your Pokémon very well, and generally only use Rare Candies when you've gotten to such a high level that reaching the last levels is a taxing process. By that time, you should have maxed your bonus stats (unless you caught them at a particularly high level).

Remember though, if you're playing Pokemon Blue, your EV cap is much higher than in the more modern games.
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Raven Dreamer♦Jun 15 '11 at 14:49

1

@Raven To my expectation, by the time you hit level 90 or so from a level 30 or under Pokémon, you probably will have maxed your EVs. I don't actually know, though.
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Grace Note♦Jun 15 '11 at 14:53

4

Don't forget the box trick - you can continue to train them after they reach level 100, and still get the full effect of the EVs gained.
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Michael MadsenJun 15 '11 at 15:06

@Ull I was referring to a different part of the system, but I guess that change in 3rd generation isn't really relevant, so I'd say I'd keep that change.
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Grace Note♦Jun 15 '11 at 16:14

@Raven: That is true, but you also got many more EVs when battling - equal to the Pokemon's base stats in fact. By my calculations you actually get a higher proportion of your maximum EVs from each battle in Generation I-II (~0.3%) than you do from Generation III onwards (~0.6%).
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DisgruntledGoatJun 15 '11 at 16:35

Rare Candies automatically boost your Pokemon to the next level. Since, at higher levels, it takes more experience to advance to the next level, using a rare candy on a high level Pokemon maximizes the benefit.

I often save and use Rare Candies for specific tasks or objectives. I do so because once a Pokémon is a high enough level there are far fewer compelling reasons to level the Pokémon. (E.G., if yours is a level 90 and you rarely encounter any Pokémon in the game above level 40 or 50, what's the point?)

As an example, I'm currently planning on using a couple to level a Pokémon exactly to level 40 so that I can use a repel and catch a roaming Pokémon at that level while not being harrassed by anything lower than that.

Personally, I used it as a way to revive PokéMon when I would run out of revives. As stated above, using Rare Candies will not level up the stats as much as if you used the PokéMon to fight.

As an example, going through Victory Road the first time, I had about 15 Revives, and a few Rare Candies. In battle, I'd revive with the Revive potion (since it also gives back 1/2 the HP). Because you can't level up during the fighting stage of a battle, you need to use a Revive. After the battle, if most of my PokéMon are fainted, I could use a Rare Candy, then either a Hyper Potion or Full Restore on them to get their HP up to the max.

I realize this isn't the sanctioned way of using Rare Candies, but it's what I do.